1. Field of the Invention
This invention relates to an emulsifier for producing microcapsules, a process for producing microcapsules by using said emulsifier, microcapsules obtained from said process, and non-carbon pressure-sensitive copying paper using said microcapsules.
2. Related Art Statement
Microcapsule is a very fine case made of a thin film and containing a liquid, solid or gaseous body in the form of fine particles measuring one to several hundreds of microns. Microcapsules for various materials such as colorless or color dyes, medicines, agricultural chemicals, perfumes, livestock feed, etc., have been industrially produced and are commercially available.
Use of such microcapsules for pressure-sensitive copying paper is well known. Such pressure-sensitive copying paper comprises an upper sheet of paper having a support coated on its backside with microcapsules containing a hydrophobic liquid having a colorless electron donative dye dissolved therein and a lower sheet of paper having a support coated on its surface side with a colorless electron receptive color-developing agent, said upper and lower sheets being combined so that their respective coated sides face each other. When a printing pressure is applied to the copying paper, said microcapsules are broken to release the contents thereof, causing the color forming agent and color developer contact each other to evoke a chemical reaction whereby a colored substance is formed on the surface of the lower sheet.
In the microcapsules, as described above, a thin film is formed around a substance having certain characteristic properties, so that the characteristic properties of the substance can be also kept confined in the microcapsules when not used, and the substance is taken out by breaking the microcapsules when so desired.
The following methods have been known for producing said type of microcapsules.
(1) Coacervation method using an ion complex of gelatin and anionic protective colloid. PA1 (2) Interfacial polymerization method making use of a film forming reaction at the interface between the inner and outer phases. PA1 (3) In-situ method in which a water-insoluble resin film is formed on the surfaces of oil droplets from the outer phase (aqueous phase) (Japanese Patent Publication No. 2100/85, and Japanese Patent Application Kokai (Laid-Open) Nos. 84881/78, 25277/79, 49984/79, 47139/80, 51238/81 and 177129/84). PA1 (1) Adjustment of pH, temperature and time of the reaction is troublesome, and the reaction operations are complicated. PA1 (2) As it is difficult to obtain a microcapsule slurry with a concentration higher than 20%, a large amount of moisture must be evaporated away in use for pressure-sensitive copying paper. Thus, there is much room for improvement on working rate, energy cost, etc., PA1 (3) Because of use of natural material, the film substance is subject to large fluctuation in terms of quality and cost. PA1 (4) The microcapsules produced according to this method can not keep long since they have a tendency to become rotten and condense. PA1 (1) Since the water-soluble polymeric materials used for emulsifying a hydrophobic liquid into the state of very fine droplest are relatively high in viscosity, the obtained microcapsule dispersion necessarily becomes high in viscosity, so that it is practically difficult to obtain a microcapsule slurry having a good fluidity with a solid content higher than 50%. PA1 (2) In case of using, as emulsifier, a water-soluble polymeric material with low viscosity or a water-soluble polymer reduced in viscosity by diluting it with a suitable solvent, emulsification stability of the hydrophobic liquid is lowered and there takes place particle aggregation or cohesion of the hydrophobic liquid. PA1 (3) In order to obtain a physically and chemically high film strength as well as high stability, it is necessary to apply a high-temperature reaction condition or to use a large amount of film material. Such change of reaction condition is undesirable in case of employing an encapsulation method which is sensitive to variation of conditions, and especially in the industrial production of microcapsules, the risk is high of producing defective or non-conforming products due to a slight error of the setting conditions or an unexpected variation of working conditions. Thus, this method, particularly in the above-said case, is greatly limited in its scope of industrial application. PA1 (1) Step for preparing a water-soluble polymer; PA1 (2) Step for preparing a core material; PA1 (3) Step for preparing an aminoaldehyde precondensate; and PA1 (4) Step for forming an aminoaldehyde resin.
These encapsulation methods are capable of forming the microcapsules having a dense film with excellent content holding strength and widely applied in the industries. Nevertheless, it is also a fact that these methods still have various problems in the aspects of production and quality.
For example, the coacervation method has the following problems.
The interfacial polymerization method can alleviate these problems in the coacervation method, but since the film base substances with high reactivity are reacted (at a relatively high temperature), this method is unsuited for encapsulation of the unstable materials or the materials which tend to be denatured thermally. Improvements are also required on solvent and water resistance of the products according to this method.
In the in-situ method, encapsulation of materials with various types of amino resin is proposed, and at present this method is most widely applied in the industries, but it still involves the following problems
Also, when the microcapsules obtained according to the known methods are used for non-carbon pressure-sensitive copying paper, the obtained copying paper would prove unsatisfactory in prevention of fouling of formed color by static pressure and in respect of heat resistance.